1. Field of the Invention
This invention concerns purification of water from produced oil streams. More particularly, it is concerned with a method for removing dissolved oils and sand contaminated with dissolved oils from that water.
2. Discussion of the Background of the Invention
Produced oil from oilwells often contains water. This water sometimes exists underground with the oil, and at other times the water is introduced into the well when formations are washed out to produce a well. It is necessary for the water to be removed from the oil, and many prior art processes have been developed to effectuate this removal.
Oil skimmers are known which can reduce the water content of the oil from a 1:1 water/oil emulsion to about five hundred parts per million (ppm) of oil. The Environmental Protection Agency, however, requires that water being discharged from an oil production facility contain only thirty ppm oil. Accordingly, additional cleaning is required.
An example of a separator system useful in reducing the oil content of the water is found in U.S. Pat. No. 4,094,783 issued to Jackson for a "Centrifugal Flotation Separator", the disclosure of which is incorporated by reference herein. This patent shows a closed cylindrical vessel with a baffle positioned inside the vessel near the top thereof, the baffle having an axial opening and wall for the upward flow of froth and gas bubbles. Contaminated liquid is introduced tangentially into the vessel so that the entering contaminated liquid, which has been mixed with air in a previous step, is circulated in a swirling motion around the inner wall of the vessel. This swirling movement provides enough residence time for the air bubbles to have an opportunity to move centripetally toward the center of the tank and rise through the axial opening in the baffle, carrying oil droplets with them. As the bubbles and droplets move through the opening in the baffle, the foam they have formed is broken and gas is released to be recycled into to the inlet liquid line. A layer of oil formed on the surface of the water can thereafter be skimmed off.
A flotation cell, such as the one shown in U.S. Pat. No. 4,094,783 patent described above, can be used to reduce the oil content of produced water. Such flotation devices are capable of removing, however, only free oil contained in the water/oil emulsion. The flotation cells using air as a flotation gas are not very useful in removing dissolved oil particles from the water.
Dissolved oil can be defined as a droplet of oil surrounded by water molecules that capture it and keep it in suspension. Dissolved oil droplets are probably, at least in part, the result of chemical additives which are introduced into the well. These chemical additives are such things as anti-corrosives having a hydrocarbon structure with an attached nitrogen group, the nitrogen atom providing extra electrons for attaching the hydrocarbon additive to the metal parts of a well structure such as a pipeline wall. The corrosion inhibitors are eventually washed off the pipeline wall and are produced with the oil. The nitrogen containing anti-corrosive molecule is then solvated by the water molecules, making them difficult to remove by conventional methods. For this reason, prior art water treatment methods have not been able to reduce the oil content of the water below about thirty parts per million.
Accordingly, it is an object of the present invention to provide a method of removing the dissolved oil to reduce oil content of the produced water to thirty parts per million or below.